Submicron Cemented Carbide with Mixed Carbides

ABSTRACT

A cemented carbide body is 1-30% by mass of binder consisting of Co, Co/Ni, Co/Fe, Co/Ni/Fe or Ni/Fe and a hard material having a hexagonal WC phase and having a face-centered cubic phase of the form (M 1 , M 2 , M 3 )C or (M 1 , M 2 , M 3 )(C, N) or (M 1 , M 2 , M 3 )(O, C, N) where M 1 =Ti and/or Zr and M 2 =W and M 3  optionally means none or one or a plurality of the elements Ta, Nb, Hf, Cr, Mo or V, wherein the proportion of the face-centered cubic phase based on the total mass is 2% to 97%, preferably 5 to 12% by mass, and the microstructure of the hexagonal phase and of the face-centered cubic phase has a mean grain size of between 0.2 μm and 1 μm, preferably ≦0.9 μm, and the mean grain sizes of the hexagonal phase and of the face-centered cubic phase differ at most by 30%.

BACKGROUND OF THE INVENTION

The invention relates to a cemented carbide body comprising 1-30% by mass of binder consisting of Co, Co/Ni, Co/Fe, Co/Ni/Fe or Ni/Fe and a hard material.

The invention also relates to a method for producing such a cemented carbide body by powder metallurgy, in which method a starting powder mixture is mixed, ground, pre-compacted to form a green compact, and finally sintered.

Cemented carbide bodies having a binder and a hard material phase, which comprise carbides, nitrides, carbonitrides or oxycarbonitrides of the elements Ti, Zr, Hf, V, Nb, Ta, Cr, Mo and/or W, are known in principle according to the prior art. It is likewise known that the hardness and the wear resistance of a cemented carbide body can be influenced by the grain size and the grain size distribution in the fully sintered body. Decisive factors are, inter alia, the composition of the starting materials, the production conditions, in particular the grain sizes of the starting powder, the grinding thereof and also the sintering temperature. The carbon balance of the cemented carbide batch also plays a considerable role when producing cemented carbide bodies by powder metallurgy.

Furthermore, it is known according to the prior art to produce cemented carbides by reaction sintering, for example by using W and Co and also carbon (C) as the starting mixture and only forming the WC hard material skeleton during the sintering process.

It is an object of the present invention to specify a cemented carbide body which has a relatively high hardness and wear resistance. Furthermore, it is an object of the present invention to specify a method for producing such a body.

SUMMARY OF THE INVENTION

The basic principle of the present invention is that the production of the cemented carbide body by powder metallurgy with final sintering is carried out on the basis of a powder starting mixture containing pre-alloyed carbides or carbonitrides of the type (M₁, M₂, M₃)C or (M₁, M₂, M₃)(C, N) or (M₁, M₂, M₃)(O, C, N) where M₁=Ti and/or Zr and M₂=tungsten and M₃ optionally means none or one or a plurality of the elements Ta, Nb, Hf, Cr, Mo or V. It is important for the microstructure of the hexagonal phase and of the face-centered cubic phase to have a mean grain size of between 0.2 μm and 1 μm, and for the mean grain sizes of the hexagonal phase and of the face-centered cubic phase to differ at most by 30%.

Various aspects of this invention will become apparent to those skilled in the art from the following detailed description of the preferred embodiment, when read in light of the accompanying drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The production of cemented carbide bodies having a hard material phase, which has both a face-centered cubic and a hexagonal phase, wherein the proportion of the face-centered cubic phase based on the total mass is between 2% by mass and 97% by mass, preferably 5 to 12% by mass, and the face-centered cubic phase has the form (M₁, M₂, M₃)C or (M₁, M₂, M₃)(C, N) or (M₁, M₂, M₃)(O, C, N) where M₁=Ti and/or Zr and M₂=W and M₃ (optionally) means none, one or a plurality of the elements Ta, Nb, Hf, Cr, Mo or V, is likewise carried out on the basis of the above-mentioned pre-alloyed phase of (M₁, M₂, M₃)C or (M₁, M₂, M₃)(C, N) or (M₁, M₂, M₃)(O, C, N) in conjunction with a hexagonal phase of WC.

The face-centered cubic phase preferably additionally contains at least one of the elements Ta, Nb, Hf, Cr, Mo or V. Chromium and vanadium, in particular, serve here as grain growth inhibitors.

According to a further configuration of the invention, the face-centered cubic phase consists of a carbonitride having a proportion of molar C of between 80% and 40% and a proportion of molar N of between 20% and 60%.

According to a further development of the present invention, the cemented carbide body has an edge zone which is free from face-centered cubic carbides or carbonitrides. A method for producing such an edge zone depleted of mixed carbides is described in DE 197 52 298 C1.

By virtue of a targeted aftertreatment, known from DE 198 45 376 A1, it is possible to provide for the formation of an edge zone, in which:

-   -   a) a carbonitride phase which is substantially free from binder         phase, preferably completely free from binder phase, is present         in an outer, first layer which adjoins the body surface and         extends down to a depth of between 2 μm and 30 μm, said         carbonitride phase;     -   b) being adjacent to an underlying middle layer having a         thickness of 5 μm to 150 μm, which consists of a substantially         pure WC—Co composition; and     -   c) in a third, lowermost layer having a thickness of at least 10         μm and at most 650 μm, the proportions of the binder phase and         of the IVa and/or Va elements change by increasing to the         substantially constant value present in the interior of the         body, and the proportion of tungsten decreases to the         substantially constant value present in the interior of the         body.

Alternatively, and without the sharp delineations of the above-mentioned layers, it is also possible to provide for the formation of an edge zone, in which:

-   -   a) in an outer layer which adjoins the body surface or an edge         zone having a depth of penetration of 1 μm to at most 3 μm and         extends down to a depth of between 10 μm and 200 μm, in the hard         material phase, the proportion of tungsten and of the binder         phase is at most 0.8 times the proportion arising from the         overall composition, and in this layer the proportion of         tungsten and of the binder phase increases substantially         continuously toward the interior of the body and the proportion         of nitrogen decreases substantially continuously toward the         interior of the body;     -   b) in an underlying middle layer having a thickness of between         20 μm and 400 μm, the tungsten and binder phase contents pass         through a maximum with an increasing depth of penetration, and         the contents of the elements of group IVa and/or Va of the         periodic table run through a minimum; and     -   c) in a third, lowermost layer which extends down to a depth of         penetration up to at most 1 mm, as measured from the body         surface, the proportions of tungsten and binder phase decrease         to substantially constant values in the interior of the body,         and the contents of elements of group IVa and/or Va of the         periodic table increase to substantially constant values.

According to a further configuration of the invention, it is likewise possible to produce edge zones close to the surface by means of a subsequent heat treatment, in which zones a further, second layer having a thickness of 2-40 μm follows underneath a first layer having a thickness of 2-100 μm and an increased proportion of binder and a reduced proportion of mixed carbides, said second layer having a higher proportion of nitrogen than the first layer and consisting essentially of nitrides and/or carbonitrides of the metals from group IVa of the periodic table and having phase proportions of up to 10% by volume of carbides, nitrides, carbonitrides or oxycarbonitrides of the elements W, Mo, V, Ta, Nb, Cr and/or proportions dissolved in the hard material phase of up to 5% by mass of V, Nb, Ta and up to 2% by mass of Cr, Mo, W and containing up to 15% by mass of binder. A transition zone having a thickness of 2 μm to 100 μm is formed underneath said second layer, in which transition zone the composition gradually changes to a homogeneous composition in the interior of the core of the cemented carbide body. In the first layer, there is therefore a relatively tough and abrasion-resistant zone having a high proportion of WC, whereas there is a diffusion-resistant, hard layer having a high proportion of nitrides or carbonitrides in the second, underlying layer. Such a layer sequence is known in principle from WO 2005/0026400 A1. In order to produce the above-mentioned cemented carbide body, use is made of a method in which, in addition to the binder metal or the binder metals, the starting mixture contains a pre-alloyed phase of the type (M₁, M₂, M₃)C or (M₁, M₂, M₃)(C, N) or (M₁, M₂, M₃)(O, C, N) where M₁=Ti and/or Zr and M₂=W and M₃ (optionally) means one or a plurality of the elements Ta, Nb, Hf, Cr, Mo or V. This starting mixture is ground, initially pressed to form a green compact and finally sintered.

For the production of a pure cemented carbide body, the starting mixture additionally contains hexagonal WC or a tungsten-containing phase, such as pure W, W₂C or WO₃, which reacts by the take up of carbon to form hexagonal WC.

Within the context of the present invention, it is possible to modify the method, in particular, in such a manner that the green compact is subjected to reaction sintering, wherein nitrogen is present in the gas atmosphere during the heating between 1100° C. and 1300° C. and, in accordance with the reaction equation:

a(M₁, M₂, M₃)C+b W+c N₂ →d(M₁, M₂, M₃)(C, N)+e WC,

forms the desired carbonitride and carbide phases, where M₁=Ti and/or Zr, M₂=W and M₃ (as optional component) means one or a plurality of the elements Ta, Nb, Hf, Cr, Mo and/or V and a, b, c, d and e are the stoichiometric coefficients. In this reaction sintering, too, the use of a pre-alloyed starting mixture reduces the grain growth considerably and improves the hardness and wear performance.

Nitrogen is fed to the sintering process such that a hard material skeleton having at least one face-centered cubic phase is formed. Depending on the proportion of W in the face-centered cubic phase or the face-centered cubic phases and in the admixed powder, and also the level of the nitrogen pressure and the duration of action of the nitrogen, the hard material skeleton of the fully sintered body may contain more or less hexagonal WC. The content of metals from group IVa, Va and/or VIa of the periodic table in the face-centered cubic phase must not correspond to the content of the fully sintered cemented carbide body.

The hard material skeleton may have a nonmetal/metal proportion ratio of 0.8 to 1.0, where the proportion of nonmetals denotes the sum of all nonmetals in the individual phases of the hard material skeleton (C+N+O) in mol and the proportion of metals denotes the sum of all metals in the phases of the hard material skeleton in mol. Depending on the choice of feed materials, the nitrogen pressure and the duration of action of the nitrogen, the coefficients in the reaction equation given above are therefore different. It is possible to influence microstructure optimization and also the setting of a specially formed binder phase additionally by adding carbon, carbides, oxides, oxycarbides, oxycarbonitrides of the metals Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W or carbonitrides to the cemented carbide batch or by feeding in carbon-emitting gases during the dewaxing, pre-sintering or sintering process.

Cemented carbides produced using the feed materials and methods described have a finer microstructure and, in some cases, a considerably greater hardness given an approximately identical fracture toughness K_(IC) and are superior to conventional cemented carbides having the same overall composition which are not subjected to reaction sintering and are produced from powders which are not pre-alloyed.

Further method variants arise by virtue of the fact that the green compact is heated in vacuo, preferably at 8° C./min, up to 1100° C., that N₂ is admitted at a pressure of between 1 Pa and 10⁷ Pa and the temperature of 1100° C. is retained for 15 to 30 min, before the green compact is heated further to 1300° C., preferably at a heating rate of 3° C./min, that the temperature of 1300° C. is retained preferably for 15 min and the body is heated further up to the sintering temperature, preferably of 1450° C., the sintering temperature is retained for 30 min and the sintered body is finally cooled to room temperature, preferably at a cooling rate of 4° C./min.

According to a further configuration of the invention, the nitrogen atmosphere is also maintained during cooling at least until 1000° C. is reached.

According to a further configuration of the invention, and in order to provide an edge zone microstructure described above, the method can be extended to the effect that, once cooling to at least 1000° C. has occurred, the sintered body is additionally treated in a nitrogen atmosphere at a nitrogen pressure of 5×10³ Pa to 10⁷ Pa at temperatures of between 1000° C. and a temperature below the eutectic, preferably below 1200° C.

According to a further configuration of the invention, it is possible, in particular, for at least some of the nitrogen to be introduced by nitrides, carbonitrides and/or oxycarbonitrides of at least one of the metals V, Nb, Ta, Cr, Mo and W which are present in the starting mixture, or by means of solid carbonitrides.

It is preferable to resort at least partially to pulverulent hard materials which have been obtained from recycled cemented carbide bodies in the pulverulent starting mixture.

In the text which follows, the invention is explained with reference to specific exemplary embodiments.

EXAMPLE 1

A mixture was produced from a pulverulent starting mixture comprising 8.1% by mass of a polyphase, pre-alloyed (Ti, Ta, Nb, Cr, W) carbide powder having low proportions of WC and W₂C, which originates from a high-temperature synthesis step, with 3.5% by mass of W, 6% by mass of Co, remainder WC. Said mixture was pressed to form a cemented carbide green compact, which was then heated to 1100° C. in a vacuum sintering furnace at 8° C./min. After a retention time of 15 minutes, a nitrogen atmosphere at a pressure of 2.5 to 2.75×10⁴ Pa was established and the body was heated to 1450° C. at 3° C./min, interrupted by a retention time of 15 minutes at 1300° C. The maximum temperature of 1450° C. was retained for 30 min, after which the body was cooled to room temperature at a cooling rate of 4° C./min. The sintered bodies had a hardness HV30 of 1840, a fracture toughness K_(IC)=9.3 MNm^(−3/2), a coercitive field strength H_(c)=25.1 kAm⁻¹ and a denitrified edge zone having a thickness of 12 μm. The average nitrogen content of the cemented carbide body was 0.3% by mass. Compared to cemented carbide bodies which have an identical overall composition but have been produced using hard material powders which are not pre-alloyed, it was possible to increase the hardness HV30 from 1560 to the indicated value of 1840.

EXAMPLE 2

A green compact was produced using a powder mixture comprising 18.8% by mass of a polyphase, pre-alloyed carbide powder of the type (Ti, Ta, Nb, Cr, W)C having low proportions of WC and W₂C, which originated from a synthesis process, and 8% by mass of Co, 6.2% by mass of W, remainder WC. The initially pressed green compact was heated to 1100° C. in a vacuum sintering furnace in vacuo at a heating rate of 8° C./min. This temperature was retained for 15 min, after which a nitrogen pressure of 2.5 to 2.75×10⁴ Pa was established. The body was then heated to 1300° C. at a further heating rate of 3° C./min, this temperature was retained for 15 min, before the body was heated to 1450° C. at the same heating rate. This temperature of 1450° C. was retained for 30 min, after which the sintered body was cooled to room temperature at a cooling rate of 4° C./min. The sintered body obtained had a hardness HV30 of 1730, a fracture toughness K_(IC) of 9.4 MNm^(−3/2), a coercitive field strength H_(c) of 23.1 kAm⁻¹ and a denitrified edge zone having a thickness of 11 μm. The average nitrogen content of the cemented carbide body was 0.5% by mass. In a comparative test, a sintered body having the same overall composition was produced, although only those hard material powders which were not pre-alloyed were used in the starting mixture. Such a sintered body had a hardness HV30 of 1510.

EXAMPLE 3

26.3% by mass of a hard material powder, made up of two phases (Ti, Ta, Nb, Cr, W)C, were mixed with 10% by mass of Co, 27% by mass of W, remainder WC, pressed to form a cemented carbide green compact and heated to 1100° C. in a vacuum sintering furnace in vacuo at 8° C./min. At the end of a retention time of 15 minutes, a nitrogen pressure of 2.5 to 2.75×10⁴ Pa was established and the body was heated to 1450° C. at a heating rate of 3° C./min, interrupted by a further retention time of 15 minutes at 1300° C. The maximum temperature of 1450° C. was retained for 30 min, after which the body was cooled to room temperature at a cooling rate of 4° C./min. The sintered bodies had a hardness HV30=1790, a fracture toughness K_(IC)=8.7 MNm^(−3/2), a coercitive field strength H_(c)=28 kAm⁻¹ and an edge zone enriched with mixed carbonitrides. The average nitrogen content of the hard material body was 2% by mass.

A sintered body having the same overall composition but produced using hard material powders which are not pre-alloyed merely had a hardness HV30=1580.

EXAMPLE 4

In a further exemplary embodiment, 37% by mass of a hard material powder, made up of two phases (Ti, Ta, Nb, Cr, W)C and originating from a synthesis process, were mixed with 25% by mass of a recycling powder, consisting essentially of WC and Co, 27% by mass of W, 8% by mass of Co, remainder WC, and pressed to form a cemented carbide green compact, which was then heated to 1100° C. in a vacuum sintering furnace in vacuo at 8° C./min. At the end of a retention time of 15 minutes, nitrogen was admitted at a pressure of 2.5 to 2.75×10⁴ Pa and the body was then heated to 1450° C. at a rate of 3° C./min, interrupted by a further retention time of 15 minutes at 1300° C. The maximum temperature of 1450° C. was retained for 30 min, after which the body was cooled to room temperature at a cooling rate of 4° C./min. The sintered bodies had a hardness HV30=1760, a fracture toughness K_(IC)=8.7 MNm^(−3/2), a coercitive field strength H_(c)=25.2 kAm⁻¹ and an edge zone enriched with mixed carbonitrides. The average nitrogen content of the hard material body was 2% by mass. Compared to a cemented carbide body having the same overall chemical composition, but produced using a starting powder mixture containing no pre-alloyed hard material powders, it was possible to increase the hardness HV30 from 1580 to 1760.

The principle and mode of operation of this invention have been explained and illustrated in its preferred embodiment. However, it must be understood that this invention may be practiced otherwise than as specifically explained and illustrated without departing from its spirit or scope. 

1. A cemented carbide body comprising 1-30% by mass of binder consisting of Co, Co/Ni, Co/Fe, Co/Ni/Fe or Ni/Fe and a hard material having a hexagonal WC phase and having a face-centered cubic phase of the form (M₁, M₂, M₃)C or (M₁, M₂, M₃)(C, N) or (M₁, M₂, M₃)(O, C, N) where M₁=Ti and/or Zr and M₂=W and M₃ optionally means none or one or a plurality of the elements Ta, Nb, Hf, Cr, Mo or V, wherein the proportion of the face-centered cubic phase based on the total mass is 2% to 97%, preferably 5 to 12% by mass, and the microstructure of the hexagonal phase and of the face-centered cubic phase has a mean grain size of between 0.2 μm and 1 μm, preferably ≦0.9 μm, and the mean grain sizes of the hexagonal phase and of the face-centered cubic phase differ at most by 30%.
 2. The cemented carbide body as claimed in claim 1, characterized in that the face-centered cubic phase contains a carbonitride having a proportion of molar C of between 80% and 40% and a proportion of molar N of between 20% and 60%.
 3. The cemented carbide body as claimed in claim 1, characterized by an edge zone which is free from face-centered cubic carbides or carbonitrides.
 4. The cemented carbide body as claimed in claim 1, characterized in that: a) a carbonitride phase which is substantially free from binder phase, preferably completely free from binder phase, is present in an outer, first layer which adjoins the body surface and extends down to a depth of between 2 μm and 30 μm, said carbonitride phase; b) being adjacent to an underlying middle layer having a thickness of 5 μm to 150 μm, which consists of a substantially pure WC—Co composition; and in that c) in a third, lowermost layer having a thickness of at least 10 μm and at most 650 μm, the proportions of the binder phase and of the IVa and/or Va elements increase to the substantially constant value present in the interior of the body, and the proportion of tungsten decreases to the substantially constant value present in the interior of the body.
 5. The cemented carbide body as claimed in either of claim 1, characterized in that: a) in an outer layer which adjoins the body surface or an edge zone having a depth of penetration of 1 μm to at most 3 μm and extends down to a depth of between 10 μm and 200 μm, in the hard material phase, the proportion of tungsten and of the binder phase is at most 0.8 times the proportion arising from the overall composition, and in this layer the proportion of tungsten and of the binder phase increases substantially continuously toward the interior of the body and the proportion of nitrogen decreases substantially continuously toward the interior of the body; b) in that in an underlying middle layer having a thickness of between 20 μm and 400 μm, the tungsten and binder phase contents pass through a maximum with an increasing depth of penetration, and the contents of elements of group IVa and/or Va of the periodic table run through a minimum; and c) in that in a third, lowermost layer which extends down to a depth of penetration up to at most 1 mm, as measured from the body surface, the proportions of tungsten and binder phase decrease to substantially constant values in the interior of the body, and the contents of elements of group IVa and/or Va of the periodic table increase to substantially constant values.
 6. The cemented carbide body as claimed in claim 1, characterized in that: a) a first layer having a thickness of 2-100 μm is provided, having a proportion of binder metal of 2-25% by mass and comprising up to 25% by volume of nitrides or carbonitrides of one or a plurality of metals from group IVa of the periodic table and/or up to 10% by volume of carbides and/or carbonitrides of V, Nb, Ta and/or Cr, remainder WC; b) a second layer which has a thickness of 2 to 40 μm and has a higher proportion of nitrogen than in the first layer is arranged underneath the first layer, said second layer consisting essentially of nitrides and/or carbonitrides of the metals from group IVa of the periodic table and containing proportions of up to 10% by volume of the elements W, Mo, V, Ta, Nb, Cr and up to 15% by mass of binder; and c) in that a transition zone having a thickness of 2 to 100 μm is arranged underneath the second layer, in which transition zone the composition gradually changes to a homogeneous composition in the interior of the core of the cemented carbide or cermet body.
 7. A method for producing a cemented carbide body comprising 1-30% by mass of binder, which consists of Co and/or Ni, if appropriate additionally Fe, remainder hard material phase, consisting of at least one metal carbide or metal carbonitride, by mixing, grinding and initially pressing a green compact and finally sintering, characterized in that, in addition to the binder metal or the binder metals, the starting powder mixture contains a pre-alloyed phase of the type (M1, M2, M3)C or (M1, M2, M3)(C, N) or (M1, M2, M3)(O, C, N) where M1=Ti and/or Zr and M2=W and M3 optionally means none or one or a plurality of the elements Ta, Nb, Hf, Cr, Mo or V.
 8. The method as claimed in claim 7, characterized in that the pulverulent starting mixture for producing a cemented carbide body additionally contains hexagonal WC or a tungsten-containing phase, such as pure W, W2C or WO3, which reacts by the take up of carbon to form hexagonal WC.
 9. The method as claimed in claim 7, characterized in that the initially pressed green compact is subjected to reaction sintering, in which N2 is present in the gas atmosphere during the heating between 1100° C. and 1300° C. and, in accordance with the reaction equation a(M1, M2, M3)C+b W+c N2→d(M1, M2, M3)(C, N)+e WC, forms the desired carbonitride and carbide phases, where M1=Ti and/or Zr, M2=W and M3 (optionally) means one or a plurality of the elements Ta, Nb, Hf, Cr, Mo or V and a, b, c, d and e are the stoichiometric coefficients.
 10. The method as claimed in claim 8, characterized in that the green compact is heated in vacuo, preferably at 8° C./min, up to 1100° C., then N2 is admitted at a pressure of between 1 Pa and 107 Pa and the temperature of 1100° C. is retained for 15 to 30 min, before the green compact is heated further to 1300° C., preferably at a heating rate of 3° C./min, in that the temperature of 1300° C. is retained preferably for 15 min and the body is heated further up to the sintering temperature, preferably of 1450° C., the sintering temperature is retained for 30 min and the sintered body is finally cooled to room temperature, preferably at a cooling rate of 4° C./min.
 11. The method as claimed in one of claim 7, characterized in that the N2 atmosphere is also maintained during cooling at least until 1000° C. is reached.
 12. The method as claimed in one of claim 7, characterized in that, once cooling to at least 1000° C. has occurred, the sintered body is additionally treated in a nitrogen atmosphere at an N2 pressure of 5×103 Pa to 107 Pa at temperatures of between 1000° C. and a temperature below the eutectic, preferably below 1200° C.
 13. The method as claimed in one of claim 7, characterized in that at least some of the nitrogen is introduced by nitrides, carbonitrides and/or oxycarbonitrides of at least one of the metals V, Nb, Ta, Cr, Mo and W which are present in the starting mixture, or by means of solid carbonitrides.
 14. The method as claimed in one of claim 7, characterized in that pulverulent hard materials which have been obtained from recycled cemented carbide bodies are present at least partially in the starting powder mixture. 